1. Technical Field
The invention relates to time-division multiple-access (TDMA) radio communication systems, and, more particularly, to communication between portable units of a TDMA radio communication system.
2. Description of the Prior Art
Radio communication is an integral part of mobile voice and data communication networks. After a decade of research and development, technical standards are being translated into commercially viable products and networks, such as Second Generation Cordless Telephone (CT2), Digital European Cordless Telephone (DECT), and Group Special Mobile (GSM) as described in W. H. W. Tuttlebee, Cordless Personal Communications, IEEE Communications Magazine, December 1992, pp. 42-53.
Many radio communication networks utilize a TDMA scheme that allows a plurality of portable units to communicate with a single base station. The base stations typically provide an interface to an external communication network. Yet, there are drawbacks in the majority of today's TDMA systems. Most systems work in a base-to-portable mode in which a portable unit has to utilize a base station and its physical channels to communicate with another portable unit even though they are within direct reach of one other. The base-to-portable mode places an undue burden on the base station and renders the portable units useless without the base station. Some radio communication systems, such as wireless local-area networks (LANs), provide direct communication among portable units utilizing standard LAN techniques, but do not use the base station channel for initial communication setup.
Recently, the Japanese Personal Handy Phone System (PHS) has been developed. PHS allows portable units to perform direct communication when communication cannot be performed via a base station. Direct communication between an origination side portable unit and a destination side portable unit is established using unstable cyclic channel scanning. The origination side portable unit detects a free transmit and receive time slot in a communication channel and calls the destination side portable unit in the free transmit time slot. The destination side portable unit periodically scans all channels searching for the calling signal transmitted by the origination side portable unit. Upon detecting the calling signal, the destination side portable unit transmits an acknowledgement signal in the receive time slot. Communication then commences between the portable units during the transmit and receive time slots. Such a method is inefficient because the portable units consume large amount of power when routinely scanning all of the channels of the system for connection to an origination side portable unit.
As radio communication systems grow to include more portable units within direct reach of one another, it is likely that the demand for direct communication between such portable units will grow as well. For example, applications such as wireless file transfer between computers, remote terminal emulation, wireless business card exchange, and cordless phone systems will require direct communication between portable units. In these applications, radio communication systems must carry both voice and data efficiently between portable units.
Accordingly, it is an object of the present invention to provide a method for efficient communication of voice and data between portable units of a TDMA radio communication system.
It is a further object of the present invention to provide a method of direct communication between portable units that conserves power and time consumed by the portable units to establish the direct communication link.
Another object of the present invention is to provide a method of direct communication between portable units that does not impact the standard protocols for communicating between portable units and the base station.
Additional objects and advantages of the invention will become apparent in light of the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.